Derivation
This tree illustrates the derivation of the ketose monosaccharides from the ketotriose D-Erythulose. When a hydroxymethylene (-HCOH) group is inserted after the carbonyl (-C=O), the next level is derived. Thus C2 becomes C3, C3 becomes C4, C4 becomes C5, etc.
ketose derivation - HCOH insertion
The diagram below shows how to identify the configuration of monosaccharides at their chiral centers via fischer projection. With the carbon skeleton aligned vertically, extending into the page, with the aldehyde group towards the top, examine the horizontal substituents. The direction of the hydroxyl group identifies the center's configuration.